1. Field of the Invention
This invention relates to biasing, and in particular to a bias reference for an amplifier.
2. Related Art
Amplifiers are used in an enormous array of applications. In many applications, amplifiers do not need to strictly adhere to rigorous performance criteria. In other applications, however, amplifiers are critical components and must function in accordance with precise operating specifications. Furthermore, market competition and customer specifications are a significant driving faction of key amplifier parameters. For example, a product with a more efficient amplifier can offer extended battery life, and thereby provide a competitive edge in the marketplace.
Thus, on one hand, the amplifiers are preferably designed and biased to draw as little battery current as possible so that battery life is extended. On the other hand, the amplifiers must also operate at a bias point that provides ample capability for generating a powerful signal with minimal adjacent channel interference.
Adjacent Channel Power Rejection (ACPR) is a measure of interference between adjacent channels. The ACPR is generally regarded as the power ratio in a wanted signal and unwanted distortion measured over the signal band. ACPR tends to be directly related to the non-linearity of the amplifier, which, to some degree, is related to the quiescent amplifier bias.
In the past, biasing techniques required a regulated voltage input for biasing the amplifier. However, the regulated voltage input, even though regulated, still was susceptible to significant temperature variation. The temperature variation caused fluctuation in the amplifier bias and thus allowed significant variation in ACPR. Alternatively, a bandgap referenced CMOS current source was sometimes used to maintain a temperature independent bias without the need for a reference voltage.
Unfortunately, substantial cost, complexity, package size constraints, and manufacturing disadvantages accompanied creation of a separate CMOS die for the current source. For example, when the amplifier itself was fabricated on a GaAs substrate, the CMOS die had to be separately designed and manufactured, allocated room in the same package with the GaAs die, and subjected to a manufacturing process with bond wires or other connections between the CMOS die and the GaAs die.
A need has long existed for an improved amplifier bias reference that addresses the problems noted above and others previously experienced.